Day
A day is Earth's full rotation relative to the Sun.
It is a fundamental unit for organizing human activity, calendars, and biological rhythms, with definitions varying by context—from civil and solar to sidereal and astronomical usage.
- sidereal_day_length
- 23 hours 56 minutes 4.09 seconds
- leap_seconds_1972_2022
- 27 inserted
- time_zones_in_use_as_of_2015
- 40
Lore & Background
The term 'day' derives from Old English dæġ, with cognates in Germanic languages such as Icelandic dagur, German Tag, and Dutch dag, all from Proto-Germanic *dagaz. Ancient custom varied the start of a day: some cultures began at sunrise or sunset, while modern civil convention starts at midnight (00:00 or 12:00 am). In astronomy, a day begins at noon so that a single night's observations share the same date.
Reader's Guide
The day is central to human timekeeping, organizing life into cycles of morning, afternoon, evening, and night that drive circadian rhythms in many organisms. Its definition adapts to different needs: the solar day varies up to 7.9 seconds due to Earth's orbital eccentricity, while the sidereal day (23h 56m 4.09s) is used in astronomy. Its role in calendars—solar, lunar, and decimal proposals—underscores its adaptability as a universal human concept.
Did You Know?
- A solar day can be up to 7.9 seconds longer or shorter than 24 hours due to Earth's orbital eccentricity.
- The Jewish religious calendar counts days beginning at sunset, while astronomy starts a day at noon.
- Daylight saving time creates one 23-hour and one 25-hour civil day each year in some countries.
- A sidereal day is about 4 minutes shorter than a solar day, lasting 23 hours 56 minutes and 4.09 seconds.
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